Acrylic cements are commonly used as bone cements in orthopedic applications, such as joint arthroplasty, in reconstruction procedures, as well as in minimally invasive applications such as for Bone augmentation.
Risks related to bone cement injection procedures result mainly from the open delivery of cement within the bone cavities. The most serious complications can involve cement particles entering the blood stream or the vertebral canals, or emboli caused by the displaced bone marrow, which can be lethal. As such, cement quality is a key determinant for the success of such treatments. Another limitation relates to excess pressure during the delivery through thin long needle for vertebral body augmentation. Excess pressure may cause the destruction of the delivery system or render the physician unable to deliver sufficient cement, resulting in a premature termination of the delivery process. On the other hand physicians may feel the pressure to complete the injection and due to the high pressure combined with the unpredictable curing behavior of the cement may resort to higher delivery rate, which may result in a higher filling rate. The higher filling rate shortens the window of time wherein the cement is delivered and may result in leakage in the due time. Further, excess pressure increases the risk of the monomer being expelled at a higher rate due to a filtration process. This phenomenon is known as filter-pressing and excess monomer has been reported to lead to allergic reaction and potentially the collapse of cardiovascular system functions.
Bone cements are usually formed by dispersing PMMA particles in a monomer. Once the particles and monomer are mixed together, the particles partially dissolve in the monomer and form an increasingly thick cohesive dough. This phase is often known as swelling. This phase is followed by a second phase: the polymerization of the cement, where the thick dough becomes a hard polymeric material.
Because of the aggressive chemical reaction involved in the polymerization and the dependency of both the swelling and the polymerization on specific environmental conditions such as temperature, humidity and method of mixing, the setting process and the properties of the cement used during the operation are generally highly unpredictable and poorly predictable through time, despite the widespread use of polymeric cement in orthopedic applications.
Physicians generally recognize this procedural limitation and have established subjective methods, depending on their experience and preferences, to find the appropriate time for cement delivery and to attempt to improve the outcome of the intervention. In practice, physicians often use visual measures to decide if the cement is ready for application, often describing the right consistency for delivery using terms such as yogurt-like, paste-like, toothpaste-like, dough-like, very dough-like, thick and cohesive, a moderately viscous solution, cake glaze-like or viscous. However, such qualifications are subjective and perceptual.
In the chemical industry, large-scale equipment such as calorimeters and rheometers are used to monitor the setting/solidification process of resins. Though some physicians have started using similar equipment in the operating room, these aparatii are usually large, expensive and require additional personnel and training to operate them.
Furthermore the waiting time prior to the polymerization phase is often considered too long, eight to nine minutes after mixing, for physicians who are under constraints to complete the procedure as efficiently as possible. This waiting time may cause some physicians to prematurely inject the cement into the patient increasing the risk of leakage of the monomer. On the other hand if several interventions are planned it may be desirable to delay the period between the mixing and the polymerization of some batches of the cement. Once the cement reaches adequate thickness, and the ideal application of the cement commences, physicians may desire to increase the effective working time of the doughy cement, in particular when a multilevel-injection is planned. Increasing the effective working time relieves the pressure on the physician, during the injection, and allows the physician to shift focus on the patient's safety instead of being preoccupied with the cement and the ability to deliver it. The ideal cement thickness is reached when the cement fills uniformly and doesn't require excessive pressure to be applied. An added measure of safety of the ideal cement arises from the monomer being consumed by the swelling process and does not filter-press under pressure.
Further, it desirable that the cement behavior becomes predicable and its application is reproducible and consistent.